1. Field of the Invention
The present invention relates to an image signal processing device and an image signal processing method therefor. The present invention is advantageously applicable to, e.g., noise reduction for a solid-state image pickup apparatus, digital camera, image capturing apparatus or similar apparatus dealing with image data.
2. Description of the Background Art
Today, a digital camera, for example, is loaded with a solid-state image sensor having high resolution, i.e., a great number of pixels generally referred to as megapixel. One of the problems with the megapixel digital camera is that the photosensitive area and therefore sensitivity of the individual cells is smaller, causing much random noise to be involved in an image picked up.
It is a common practice with the digital camera to execute noise reduction by using a low-pass filter. Japanese patent laid-open publication No. 235472/1992, for example, discloses an image pickup apparatus using median filter means for noise reduction. A recursive noise reducer is another conventional implementation for noise reduction. The recursive noise reducer includes a memory capable of storing more than one frame of image data. Image signals representative of one and the same scene are cumulatively stored into the memory so as to produce mean values for the pixels to thereby reduce noise.
The noise reduction by filtering, however, has a problem that the filtering lowers, e.g., the resolution of an image when constantly applied to the image, thereby degrading image quality. Further, while the filtering effects unconditional noise reduction, a particular filter must be assigned to each scene to be picked up or each shooting mode, i.e., each amount of noise. Mounting a plurality of filters on a digital camera is not practical.
The recursive noise reducer using an averaging method is effective only if a scene being picked up does not vary over the whole image capturing time. This limits the range of scenes to which the recursive noise reducer is applicable. Moreover, the memory for storing a plurality of images, coupled with a long shooting time ascribable to repeated pickup, critically impairs the function of the digital camera.
On the other hand, when a scene with a low signal level, e.g., a night scene is picked up, the digital camera increases sensitivity or otherwise corrects gain at the time of signal processing. The gain correction, however, amplifies noise together with the level of an image signal. Specifically, signal processing customary with, e.g., a digital camera generally includes nonlinear gamma correction for correcting the tonality of an image to be output. The gamma correction corrects gain more in a low luminance range than in a high luminance range. As a result, noise is enhanced at low luminance levels. In this manner, noise is dependent on shooting conditions and pixel level.